1. Field of Use
This invention relates generally to multisection telescopic booms such as used on mobile cranes or the like. In particular, it relates to means and method for extending and retracting the outermost fly section of the crane which does not have its own power means.
2. Description of the Prior Art
Various types of multisection, telescopic crane booms are known and which need to be extended to great lengths, able to handle very heavy loads, and relatively light and compact to facilitate their mobility. Accordingly, it is customary to design such cranes so that, in operation, the fly section is permanently extended and for weight and economy, to rely on means other than power means to extend and retract the fly section. U.S. Pat. No. 3,795,321 issued Mar. 5, 1974, to R. L. Johnston for "Telescoping Crane Boom" discloses a crane boom of the general type under consideration, means and a method for extending a fly section without its own power extension means, and describes a method of extending or retracting a fly section by connecting the movable portion of a hydraulic ram to the fly section. To make this connection, the crane operator must align several access and pinning holes in the various boom sections, and because these holes can only be visually aligned and the crane operator cannot see the holes when he is controlling the boom sections from the crane operator's cab, it is required that a second man position himself alongside the boom sections so that he can signal the crane operator when the holes are in alignment.
The device of the Johnston U.S. Pat. No. 3,795,321 also requires heavy pins to connect the fly section to the second section when the former is retracted or extended relative to the latter, which pins must be inserted manually. This can be difficult even for a strong individual, as the access and pinning holes on larger cranes are often several feet over his head when the boom is in its lowermost position. This necessitates the operator's climbing in a ladder or otherwise raising himself up to a level such that he can insert the pin. Furthermore, the size of the pin connecting the fly and second section or load pin is limited to the size that an operator can lift in place. In the boom described by the Johnston patent, where the load pin is inserted through the sides of the fly section and the second section and is in contact with both sections whether the fly section is extended or retracted from the second section, the load pin must transfer axial and bending loads. When the boom section is extended or retracted, the load pin must transfer all axial loads from the boom point to the rest of the boom. When the boom section is extended, the load pin must also transfer all bending loads from the boom point to the rest of the boom. Thus, in the extended or retracted position, the lift capacity of the boom is limited by the shear strength of the pin.
The stop pins, which are used in the Johnston patent to maintain the fly section's axial position relative to the base section while other movable sections are being extended or retracted, must also be inserted manually in Johnston, and are thereby limited in size because of the factors listed hereinabove which limit the size of the load pins. in order to fully extend the fly section of the Johnston's patented device, the stop pins must be inserted therein. When the fly section in Johnston has been fully extended, the load pin must be inserted to maintain the fly in this extended position while the telescopable sections are moved axially. If the stop pins are inadvertently not removed from the fly section after the load pin has connected the extended fly section and the second section, and the second section is then telescoped into the third and base section, breakage of the stop pin may result. Such breakage is more likely when the stop pins are of relatively light construction.
Finally, the stop pins and load pins of prior devices, because they may be removed from the boom, are subject to misplacement and loss.